1. Field of the Invention
This invention relates to laser packaging and, in particular, to apparatuses and methods for controlling the temperature of and electrically interfacing a laser with external circuitry.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
In semiconductor lasers, a semiconductor active region serves as the gain medium. Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, in contrast to SELs, in which the radiation output is perpendicular to the wafer surface, as the name implies.
Semiconductor lasers are used in a variety of applications, such as high-bit-rate optical fiber communications. In telecommunications applications, the laser often emits at a single lasing wavelength at 1.31 μm (and other closely spaced wavelengths), or at telecommunications wavelengths specified by the ITU grid, such as lasing wavelengths of 1.55 μm (and other closely spaced wavelengths). These wavelength ranges are often used for telecommunications purposes because the loss of silica fibers is comparatively low at these wavelengths.
Lasers must be optically coupled to fibers to engage in optical fiber communications. For example, a 1.31 μm edge-emitting laser's output must be optically coupled into the input (light-receiving) end of an optical fiber in order to transmit a modulated optical signal via the fiber.
Various modules, assemblies or packages are used to hold the laser, other optical components (such as collimation and coupling lenses, isolators, and the like), and optical fiber while being aligned with each other so as to permit the laser to be optically coupled to the fiber, i.e. light output from the laser is transmitted into the fiber. The process of aligning an optical fiber to a laser diode and fixing it in place is sometimes known as fiber pigtailing. Various laser packages (housings) are employed to align and position the laser, fiber, and related optical components to each other so the laser is optically coupled to the fiber. The housing may include intermediate optical components such as one or more lenses, isolators, and the like. The standard laser package housing types include TO (transistor outline) can and butterfly packages.
For example, the laser (usually a laser diode) and the light-receiving end of the optical fiber to which the laser is to be optically coupled may be mounted together in a TO can housing. The laser is mounted on a laser submount on the TO can post of the TO header. The fiber end may be disposed in a rigid cylindrical ferrule, which is itself mounted inside a cylindrical ferrule housing. The TO header has a substantially cylindrical portion having a lens disposed in an opening in the top portion thereof, in the TO cap, which can be disposed between the laser and the fiber end. The fiber ferrule housing is mounted to the TO can. The TO can housing may also include other related components, such as a lens and a monitor photodiode, and is typically hermetically sealed.
In metal boxlike housings (packages) such as butterfly housings, the laser and related components are mounted on a platform such as an optical bench. These related components may include laser circuitry including signal conditioning and impedance matching circuits, and a temperature sensor. The laser and laser circuitry are electrically connected to some of the 7×2 pins extending laterally from the housing. The optical bench may be mounted on a temperature stabilizing (thermal stabilization) means comprising a heat radiating element such as a Peltier effect element, i.e. a thermoelectric cooler, or TEC, inside the boxlike housing. In one approach, there is an opening in an end sidewall of the housing, through which is inserted a metal pipe (ferrule). The fiber is inserted through the ferrule into the inside of the housing, and soldered to the metal ferrule for a sealed fit. Disposed on the platform between the laser and the input end of the fiber are typically components such as collimating lens, isolator, and one or more lenses.
When a metal butterfly housing is employed, a diode laser chip on submount is typically mounted onto an optical bench in the housing. The butterfly housing is to be hermetically sealed and will contain the laser and associated optics. The optical bench may itself be mounted on a TEC inside the laser housing. The TEC is used to control the temperature of the diode laser to permit higher performance and/or operation over a greater power range. For these and other reasons, butterfly housings are typically much more expensive than TO can housings and are generally used for higher performance applications.
The butterfly housing may be mounted onto a larger circuit board, sometimes referred to as a transmitter board or motherboard, which contains external circuitry (such as drive circuitry and other circuitry) and a heatsink in contact with the TEC. The transmitter board contains a mating section designed to receive the butterfly housing. The mating section may be, for example, a rectangular opening in the transmitter board having contact points on the upper surface of the board, around the opening, located so as to be in contact with the 7×2 pins extending from the butterfly housing when it is lowered into the opening so that the pins are flush with the upper surface of the transmitter board. A heatsink is mounted to the TEC which extends through the opening.